Centripetal force Watch

nazgul60
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If the centripetal force is acting inwards, why do you feel thrown outwards when travelling around a bend in a car?
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Mr M
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(Original post by nazgul60)
If the centripetal force is acting inwards, why do you feel thrown outwards when travelling around a bend in a car?
This is a consequence of Newton's First Law. You want to continue to travel in a straight line.
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Stonebridge
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To add to what Mr M has said, there is also an element of Newton's 3rd law here.
The question is interesting because it asks about what you actually feel.
You feel the centripetal force acting on your body. In turn, your body applies an equal and opposite force on whatever is producing the centripetal force. It is this 2nd force, the Newton 3 reaction to the centripetal force, that is often described as the force you feel is "throwing you outwards" when going in a circular path.
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nazgul60
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(Original post by Mr M)
This is a consequence of Newton's First Law.
Which states that velocity remains constant unless an external force acts upon the object? Dont see the connection?
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nazgul60
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(Original post by Mr M)
This is a consequence of Newton's First Law. You want to continue to travel in a straight line.
Ok I get it!
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boromir9111
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(Original post by nazgul60)
Which states that velocity remains constant unless an external force acts upon the object? Dont see the connection?
constant = straight line in this context and external force is the centripetal force
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nazgul60
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(Original post by Stonebridge)
To add to what Mr M has said, there is also an element of Newton's 3rd law here.
The question is interesting because it asks about what you actually feel.
You feel the centripetal force acting on your body. In turn, your body applies an equal and opposite force on whatever is producing the centripetal force. It is this 2nd force, the Newton 3 reaction to the centripetal force, that is often described as the force you feel is "throwing you outwards" when going in a circular path.
Thanks
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TurboCretin
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(Original post by boromir9111)
constant = straight line in this context and external force is the centripetal force

(Original post by nazgul60)
Thanks
Loving the LOTRness of this thread.

Anyway, to answer your question with an analogy, it's like asking why we feel an upward force on our feet when gravity is acting downwards. Newton's 3rd law.
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boromir9111
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(Original post by TurboCretin)
Loving the LOTRness of this thread.
:cool:
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3nTr0pY
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(Original post by Stonebridge)
To add to what Mr M has said, there is also an element of Newton's 3rd law here.
The question is interesting because it asks about what you actually feel.
You feel the centripetal force acting on your body. In turn, your body applies an equal and opposite force on whatever is producing the centripetal force. It is this 2nd force, the Newton 3 reaction to the centripetal force, that is often described as the force you feel is "throwing you outwards" when going in a circular path.
(Original post by TurboCretin)
Anyway, to answer your question with an analogy, it's like asking why we feel an upward force on our feet when gravity is acting downwards. Newton's 3rd law.
I'm not sure I agree with either of your arguments. First off, on a more minor point, TurboCretin implies that gravity acting downwards and the upward force on your feet are a 3rd law pair. To be clear, in reality the downward reaction force of your feet on the earth is the real 3rd law pair with the upward reaction force of the earth on your feet. Of course, the force of gravity causes that.

More importantly, I'm not convinced that your 3rd law law explanation for the centrifugal force is correct, even though it would give the right answer. The body applies a centripetal force on you, but the Newton's 3rd law pair of that is you applying that same force on the body. So surely the force on you remains unchanged and thus the force you feel remains unchanged?

All in all, to me, the 3rd law explanation doesn't make any sense - I hope I haven't missed something.

For a far more clear and understandable explanation of fictitious forces such as the centrifugal force one should transform to ones own reference frame (i.e. the reference frame of the orbiting particle). Then it becomes clear why such a force is felt.
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purplesparks14
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Would 3nTrOpY care to explain further? I am confused.

And tangentially, what is the difference between linear speed and angular speed?
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Stonebridge
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(Original post by 3nTr0pY)
I'm not sure I agree with either of your arguments. First off, on a more minor point, TurboCretin implies that gravity acting downwards and the upward force on your feet are a 3rd law pair. To be clear, in reality the downward reaction force of your feet on the earth is the real 3rd law pair with the upward reaction force of the earth on your feet. Of course, the force of gravity causes that.

More importantly, I'm not convinced that your 3rd law law explanation for the centrifugal force is correct, even though it would give the right answer. The body applies a centripetal force on you, but the Newton's 3rd law pair of that is you applying that same force on the body. So surely the force on you remains unchanged and thus the force you feel remains unchanged?

All in all, to me, the 3rd law explanation doesn't make any sense - I hope I haven't missed something.

For a far more clear and understandable explanation of fictitious forces such as the centrifugal force one should transform to ones own reference frame (i.e. the reference frame of the orbiting particle). Then it becomes clear why such a force is felt.
I'm not sure which bit of this refers to my post so forgive me if I assume incorrectly. Anyway, here's clarification.
1st, I didn't mention centrifugal force. Anywhere. So my post was not an explanation of "centrifugal force". That's something you have introduced into the thread.
2nd The emboldened part of your post is exactly what I said.
I wrote "You feel the centripetal force acting on your body. In turn, your body applies an equal and opposite force on whatever is producing the centripetal force. "
3rd The original poster asked why you feel "thrown outwards". This is as much a subjective question as it is one of actual contact forces and Newton 3 reactions.
The reality is, you just "feel" the one force. The centripetal force. This force has its Newton 3 reaction, as discussed. Without this force you don't go in a circle. You can't go in a circle without physically feeling this force. The force acts towards the centre, not outwards, so you then have to ask what is this feeling of being thrown outwards. That's where the Newton 3 comes in, because that force, by you on your support/surroundings is outwards in direction.
I hope this has cleared it up.
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3nTr0pY
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I have to head off somewhere now but I'll give a brief reply (which I might expand on later)

(Original post by purplesparks14)
Would 3nTrOpY care to explain further? I am confused.

And tangentially, what is the difference between linear speed and angular speed?
Angular velocity, usually denoted omega, = v/r. Essentially, angular velocity of a particle is the amount of angle it covers per second. When doing angles, it's generally best to work with radians where pi = 180 degrees but I'm not sure how much of this sort of stuff you've covered.


(Original post by Stonebridge)
I'm not sure which bit of this refers to my post so forgive me if I assume incorrectly. Anyway, here's clarification.
1st, I didn't mention centrifugal force. Anywhere. So my post was not an explanation of "centrifugal force". That's something you have introduced into the thread.
2nd The emboldened part of your post is exactly what I said.
I wrote "You feel the centripetal force acting on your body. In turn, your body applies an equal and opposite force on whatever is producing the centripetal force. "
3rd The original poster asked why you feel "thrown outwards". This is as much a subjective question as it is one of actual contact forces and Newton 3 reactions.
The reality is, you just "feel" the one force. The centripetal force. This force has its Newton 3 reaction, as discussed. Without this force you don't go in a circle. You can't go in a circle without physically feeling this force. The force acts towards the centre, not outwards, so you then have to ask what is this feeling of being thrown outwards. That's where the Newton 3 comes in, because that force, by you on your support/surroundings is outwards in direction.
I hope this has cleared it up.
But the centrifugal force is the fictitious force that the orbiting particle experiences. The question of what you feel is definitely not a purely subjective question. Explaining exactly what I mean quickly is an impossible challenge but you can check out this in the meantime:

http://en.wikipedia.org/wiki/Centrifugal_force

If you're doing a physics degree, you'll probably study the fictitious forces that arise from various reference frames in your second year (if you haven't done it already).

Of course, the centripetal force is the only 'real' force acting on the particle but if you transform to the reference frame of the orbiting particle it feels the centrifugal force which is equal and opposite to the centripetal one.

Sorry, gotta go but will probably make another post later.
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TurboCretin
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(Original post by 3nTr0pY)
I'm not sure I agree with either of your arguments. First off, on a more minor point, TurboCretin implies that gravity acting downwards and the upward force on your feet are a 3rd law pair. To be clear, in reality the downward reaction force of your feet on the earth is the real 3rd law pair with the upward reaction force of the earth on your feet. Of course, the force of gravity causes that.

More importantly, I'm not convinced that your 3rd law law explanation for the centrifugal force is correct, even though it would give the right answer. The body applies a centripetal force on you, but the Newton's 3rd law pair of that is you applying that same force on the body. So surely the force on you remains unchanged and thus the force you feel remains unchanged?

All in all, to me, the 3rd law explanation doesn't make any sense - I hope I haven't missed something.

For a far more clear and understandable explanation of fictitious forces such as the centrifugal force one should transform to ones own reference frame (i.e. the reference frame of the orbiting particle). Then it becomes clear why such a force is felt.
The OP is asking why you feel an outward force when moving with circular motion, and I was supplying an analogous question. I didn't actually say that they were a third law pair; I wasn't saying anything so specific. I was merely drawing attention to the material point that when you stand, the force which imbues you with weight acts downward, yet you only feel the reactionary upward force.
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Stonebridge
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(Original post by 3nTr0pY)



But the centrifugal force is the fictitious force that the orbiting particle experiences. The question of what you feel is definitely not a purely subjective question
I have never mentioned "centrifugal force" other than in reference to your mention of it.
I didn't say it was "purely subjective". I said it was both. (Subjective and objective)

Explaining exactly what I mean quickly is an impossible challenge but you can check out this in the meantime:

http://en.wikipedia.org/wiki/Centrifugal_force
If you read the discussion page for this article, you will see that it is far from definitive. There are, and have always been, disputes over the meaning and interpretation of "centrifugal force" in connection with this type of scenario.
I've contributed to a number of Wiki articles on this topic. (Not under the "Stonebridge" pseudonym.)

The question wasn't about Coriolis Forces and doesn't need to be.
It was simply about what you "feel".
Fictitious forces are fine for purposes of mathematical analysis, but don't help much in answer to this question about the real forces you feel.
As we have both pointed out, there is only one force you actually feel on you. And this force has its Newton 3 reaction, which we also agree on.
So I'm at a loss as to quite what we are supposed to be disagreeing on.
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Mr M
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From Introduction to Physical Science by Shipman, Wilson and Todd:



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Clearly there is a 3rd law pair - seatbelt pressing on the driver and the driver pressing on the seatbelt but I don't think this adds anything by way of explanation.
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Stonebridge
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It depends what you are trying to explain.
In the car example the driver finds himself pushing outwards on the inside of the car and on the seat. This is the Newton 3 reaction to the centripetal force which we all agree on. The OP asked about the feeling of being thrown outwards. From the point of view of this person this outwards (Newton 3) force is the only force in that direction that he will be aware of.
It's not an explanation of the dynamics of the passenger, simply an explanation of the feeling of being thrown or pushed outwards.
Unfortunately, debates on this topic always end up like this. The wiki articles' debate pages are a classic example.
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3nTr0pY
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(Original post by TurboCretin)
The OP is asking why you feel an outward force when moving with circular motion, and I was supplying an analogous question. I didn't actually say that they were a third law pair; I wasn't saying anything so specific. I was merely drawing attention to the material point that when you stand, the force which imbues you with weight acts downward, yet you only feel the reactionary upward force.
The example you gave was correct but I was really just clarifying that it was not relevant to the discussion at hand. Whilst that analogy works in many situations I think it serves only to confuse when talking about the centrifugal force (which is what we are talking about) because it is not an example of a 3rd law pair. Indeed Newton's 3rd law has got nothing to do with this particular analysis whatsoever.


(Original post by Stonebridge)
I have never mentioned "centrifugal force" other than in reference to your mention of it.
I didn't say it was "purely subjective". I said it was both. (Subjective and objective)



If you read the discussion page for this article, you will see that it is far from definitive. There are, and have always been, disputes over the meaning and interpretation of "centrifugal force" in connection with this type of scenario.
I've contributed to a number of Wiki articles on this topic. (Not under the "Stonebridge" pseudonym.)

The question wasn't about Coriolis Forces and doesn't need to be.
It was simply about what you "feel".
Fictitious forces are fine for purposes of mathematical analysis, but don't help much in answer to this question about the real forces you feel.
As we have both pointed out, there is only one force you actually feel on you. And this force has its Newton 3 reaction, which we also agree on.
So I'm at a loss as to quite what we are supposed to be disagreeing on.
What we still disagree on is your first post - which was wrong. Let me refresh your memory:

(Original post by Stonebridge)
To add to what Mr M has said, there is also an element of Newton's 3rd law here.
The question is interesting because it asks about what you actually feel.
You feel the centripetal force acting on your body. In turn, your body applies an equal and opposite force on whatever is producing the centripetal force. It is this 2nd force, the Newton 3 reaction to the centripetal force, that is often described as the force you feel is "throwing you outwards" when going in a circular path.
Look at the bolded bit. That sentence must be wrong because the 2nd force you are describing acts on the body you are orbiting, not on yourself. Thus it is impossible that the force you are describing is the one that you are feeling.


In terms of analysis, lets make this simple:

Imagine we have a massive sun with a planet revolving around it. The sun exerts a gravitational force on the planet. The Newton's 3rd law pair to this is the gravitational force of the planet on the sun. Thus the only force on the planet is the centripetal force. The Newton's 3rd law pair is irrelevant because it acts on the sun, not on the planet.

The reason why the planet would 'feel' a force going outwards can be demonstrated by transforming to the frame of reference of the planet and can be found in any decent physics textbook on classical dynamics. A cute way to think of it though is as the force trying to drag the curved path of the orbit back into a straight line, and thus obviously it must be equal and opposite to the centripetal force. This force is the centrifugal force.


Interestingly, just by browsing through wiki I saw that your misconception is a common and very understandable one:

Another very common mistake is to state that

The centrifugal force that an object experiences is the reaction to the centripetal force on that object.

Clearly, if an object were simultaneously subject to both a centripetal force and an equal and opposite reactive centrifugal force, the resultant force would vanish and the object could not experience a circular motion. The centrifugal force is sometimes called a fictitious force or pseudo force, to underscore the fact that such a force only appears when calculations or measurements are conducted in non-inertial reference frames. However, the term centrifugal force can also be used, in a different meaning, to denote the reaction force to the centripetal force. It is correct to state, for example: A car driving in a curve exerts a centrifugal force on the road
http://en.wikipedia.org/wiki/Reaction_%28physics%29


EDIT: But just browsing through wiki, the various different definitions they have of the centrifugal force serve only to confuse things further. When I use centrifugal force I'm referring to the fictitious force.
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Stonebridge
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(Original post by 3nTr0pY)

What we still disagree on is your first post - which was wrong. Let me refresh your memory:


Look at the bolded bit. That sentence must be wrong because the 2nd force you are describing acts on the body you are orbiting, not on yourself. Thus it is impossible that the force you are describing is the one that you are feeling.


In terms of analysis, lets make this simple:
Imagine we have a massive sun with a planet revolving around it. The sun exerts a gravitational force on the planet. The Newton's 3rd law pair to this is the gravitational force of the planet on the sun. Thus the only force on the planet is the centripetal force. The Newton's 3rd law pair is irrelevant because it acts on the sun, not on the planet.

The reason why the planet would 'feel' a force going outwards can be demonstrated by transforming to the frame of reference of the planet and can be found in any decent physics textbook on classical dynamics. A cute way to think of it though is as the force trying to drag the curved path of the orbit back into a straight line, and thus obviously it must be equal and opposite to the centripetal force. This force is the centrifugal force.


Interestingly, just by browsing through wiki I saw that your misconception is a common and very understandable one:


http://en.wikipedia.org/wiki/Reaction_%28physics%29
I resent your patronising tone.
If you wish to discuss this further PM me.
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3nTr0pY
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(Original post by Stonebridge)
I resent your patronising tone.
If you wish to discuss this further PM me.
I'm not being patronising for the sake of it - it's only because you confuse the hell outta me. You said this, remember:

(Original post by Stonebridge)
So I'm at a loss as to quite what we are supposed to be disagreeing on.

Even after I'd made two posts which had clearly demonstrated what we disagree on. To simply dismiss my arguments without a proper response was, to my mind, pretty rude.

You're no doubt a smart chap so I don't see why you couldn't understand my argument the first two times around and respond accordingly. My later post only sounded patronising because you didn't get what I was saying the first two times.


EDIT: And anyway, I object:

1. Simplicity is good.
2. I like the term cute - and I like that way of thinking
3. The misconception is a very understandable one...
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